When frameworks like BlackSheep set Transfer-Encoding: chunked on
streaming responses, gunicorn was adding a second header without
checking if one already exists. This caused httpcore to reject the
response with "multiple Transfer-Encoding headers" error.
Fix checks for existing Transfer-Encoding header before adding one,
while still enabling chunked body encoding when the framework sets it.
- Add _close_sent, _close_received, _close_event state variables
- Server now waits for client's close frame response before marking
connection as closed (5s timeout)
- Update _read_frames loop to continue reading after sending close
- Fix tests to simulate client close frame response
